Translation of mRNA

ABSTRACT

The invention provides use of the 5&#39;-leader sequence of an RNA virus up to the first initiation codon of the viral RNA (or a derivative or portion thereof), either as RNA or a complementary DNA, as an enhancer of translation of mRNA when the 5&#39;- leader RNA is contiguous with the mRNA.

This application is a Continuation of application Ser. No. 08/255,796, filed on Jun. 7, 1994, now abandoned, which is a continuation of application Ser. No. 08/125,399, filed on Sep. 23, 1993, now abandoned, which is a continuation of application Ser. No. 07/948,472, filed on Sep. 22, 1992, now abandoned, which is a continuation of application Ser. No. 07/640,946, filed on Jan. 14, 1991, now abandoned, which is a continuation-in-part of application Ser. No. 07/148,650, filed on Feb. 4, 1988, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to enhancers of the translation of mRNA.

FIELD OF THE INVENTION

The mechanisms by which eukaryotes and prokaryotes initiate translation are known to have certain features in common and to differ in others. Eukaryotic messages are functionally monocistronic, translation initiates at the 5' end and is stimulated by the presence of a cap structure (m⁷ G^(5') ppp^(5') G . . . ) at this end (Shatkin, Cell 9 645 (1976)). Prokaryotic messages can be polycistronic, can initate at sites other than the 5' terminus, and the presence of a cap does not lead to translational stimulation. Both eukaryotes and prokaryotes begin translation at the codon AUG, although prokaryotes can also use GUG. Translation in both is stimulated by certain sequences near the start codon. For prokaryotes, it is the so-called Shine-Dalgarno sequence (a purine rich region 3-10 nucleotides upstream from the initiation codon). For eukaryotes, it is a purine at the -3 position and a G residue in the +4 position (where the A of the AUG start codon is designated +1), plus other sequence requirements involved in finer tuning. This is part of the "relaxed" version of the scanning model (Kozak, Nuc. Acids. Res. 12, 857 (1984)) whereby a 40S ribosomal sub-unit binds at the 5' end of the eukaryotic mRNA and proceeds to scan the sequence until the first AUG, which meets the requirements of the model, is encountered, at which point a 60S sub-unit joins the 40S sub-unit, eventually resulting in protein synthesis. Reference can be made in this connection to the following publications by Kozak: Cell 15, 1109 (1978), Nuc. Acid. Res. 9, 5233 (1981) and Cell 44, 283 (1986).

Beyond these sequence requirements in and about the initiation codon, there are no additional regions of mRNA which are known to enhance translation reproducibly.

During eukaryotic transcription, sequences known as "enhancer" regions can provide transcriptional stimulation.

RNA viruses contain either positive or negative single-stranded RNA or double-stranded RNA. The class of RNA viruses include the majority of plant viruses (over 90%), some animal viruses and several bacteriophages.

One of the most widely studied of RNA viruses is tobacco mosaic virus (referred to hereinafter as TMV). The complete nucleotide sequence of TMV is known (Goelet et al., Proc. Natl. Acad. Sci. USA 79 5818 (1982)). The 5'- region of TMV RNA was first isolated in 1965 as an RNase T₁ -resistant fragment 70 nucleotides long (Mundry, Z. Vererbungsl. 97, 281 (1965)) and free of internal G residues (Mandeles, J. Biol. Chem. 243, 3671). This region (later referred to as omega, Mandeles J. Mol. Biol. Chem. 243 3671) was sequenced (Richards et al. Eur. J. Biochem. 84 513 (1978)) and shown to form disome initiation complexes with wheat germ ribosomes. One ribosome occupies the AUG initiation site of the 126 KDa protein coding region and the second binds to the upstream leader sequence (Filipowicz and Haenni, Proc. Natl. Acad. Sci. USA 76, 3111 (1979); Konarska et al., Eur. J. Biochem. 114, 221 (1981)). Recent additional evidence to support the role of omega in ribosome association comes from the uncoating/gene expression data of Wilson and co-workers, (reviewed in J. Gen. Virol. 66, 1201 (1985) and UCLA Symp, 54 159 (1987)). It was observed in various in vitro translation systems that TMV particles are uncoated by the binding of ribosomes to the 5' end and their translational movement toward the 3' terminus.

SUMMARY OF THE INVENTION

It has now been found that the 5'- regions of RNA viruses act as enhancers of translation of mRNA.

According to a first aspect the present invention provides mRNA including the 5'- leader sequence of an RNA virus up to the first initiation codon of the viral RNA (or a derivative or portion thereof), the mRNA also including downstream sequence not derived from the virus providing the leader sequence.

According to a second aspect the invention provides a DNA sequence complementary to the 5'- leader sequence of an RNA virus, up to the first initiation codon of the viral RNA (or a derivative or portion thereof).

According to a third aspect the invention provides a DNA sequence comprising a promoter, a sequence complementary to the 5'- leader sequence of an RNA virus up to the first initiation codon of the viral RNA (or a derivative or portion thereof) and an open reading frame including an appropriate initiation codon. The invention also provides an expression vector including such a DNA sequence.

According to a fourth aspect the invention provides the use of the 5'- leader sequence of an RNA virus up to the first initiation codon of the viral RNA (or a derivative or portion thereof), either as RNA or as complementary DNA, as an enhancer of translation of mRNA when the 5'- leader RNA is contiguous with the mRNA.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Production of chimaeric mRNA containing Ω';

FIG. 2 SDS-PAGE analysis of translation products from MDL and WG;

FIG. 3 SDS-PAGE analysis of translation products from E. coli.

FIG. 4 Chimaeric RNA constructs containing mRNA and derivatives and portions of the Ω' leader sequence;

FIG. 5 Chimaeric RNA construct containing CAT mRNA with the 5' leader sequence from tomato TMV (SPS isolate);

FIG. 6 Chimaeric RNA construct containing CAT mRNA with the 5' leader sequence from TYMV;

FIG. 7 Chimaeric RNA construct containing CAT mRNA with the 5' leader sequence from BMV RNA 3;

FIG. 8 Chimaeric RNA construct containing CAT mRNA with the 5' leader sequence from RSV;

FIG. 9 Chimaenc RNA construct containing CAT mRNA with the 5' leader sequence from A1MV RNA 4;

FIG. 10 Chimaeric RNA construct containing GUS mRNA (bad context) with the 5' leader sequence from pSP64 polylinker;

FIG. 11 Chimaeric RNA construct containing GUS mRNA (bad context) with the 5' leader sequence from U1-Ω;

FIG. 12 Chimaeric RNA construct containing GUS mRNA (good context) with the 5' leader sequence from polylinker;

FIG. 13 Chimaeric RNA construct containing GUS mRNA (good context) with the 5' leader sequence from U1-Ω;

FIG. 14 Effect of Ω' on the expression of capped or uncapped CAT RNAs in electroporated tobacco protoplasts;

FIG. 15 Effect of Ω' on the expression of capped or uncapped CAT RNAs microinjected into X. laevis oocytes;

FIG. 16 Plasmids pJII6, pJII7;

FIG. 17 Chimaeric RNA constructs with the NPT II coding region and either Ω', two tandem Ω' regions, a long (77 base) "junk" leader (polylinker) sequence or "junk" together with a central Ω';

FIG. 18 Trp promoter modified to create a 3' Hind III site and a blunt 5' end;

FIG. 19 Constructs pUC35SCATNos and pUC35SCATNos and pUC35SΩ'CATNos; and

FIG. 20 Graph showing expression of CAT in constructs pUC35SCATNos and pUC35SΩ'CATNos.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Most RNA viruses which have been sequenced contain 5'-leader sequences, i.e. sequences at the 5'- end up to but not including the first initiation codon. Appropriate 5'- leader sequences from any RNA virus may be used as a translational enhancer according to the invention.

As noted above, the 5'- leader sequence of TMV plus the AUG initiation codon is known as omega and its nucleotide sequence is known (Richards et al. Eur. J. Biochem. 84, 513 (1978)) so that cDNA can be prepared synthetically. The omega prime (Ω, see FIG. 1) sequence of TMV represents the preferred translational enhancer according to the invention.

Information is also available about the translational enhancement by 5'- leader sequences of other RNA viruses including the following:

Turnip yellow mosaic virus genome RNA (Filipowicz and Haenni, Proc. Natl. Acad. Sci. USA 76, 3111 (1979)), brome mosaic virus RNA 3, alfalfa mosaic virus RNA 4 (Goelet et al., Proc. Natl. Acad. Sci. USA 79 5818 (1982)); Rous Sarcoma virus (J. L. Darlix et al. Nuc. Acids Res. 10, 5183 (1982)).

In any particular case it will almost certainly be possible to vary the natural 5'- leader sequence in such a way that its function as a translational enhancer is modulated but not destroyed. Such variants are referred to herein as "derivatives" and derivatives of the natural sequence and their use as translational enhancers form part of the present invention. It may also be the case that parts only of the natural 5'- leader sequence (or a derivative sequence) are required to provide translational enhancement. Such parts of the sequence are referred to herein as "portions" and portions of the natural sequence or of derivatives thereof and their use as translational enhancers form part of the present invention.

In any particular case, determination of the nucleotide sequence of the 5'- leader of an RNA virus should be within the expertise of a man of ordinary skill in the art using known and readily available techniques. Similarly, identification of those parts of the natural 5'- leader sequence which are essential for translational enhancement and those which can be modified or omitted (i.e. the identification of derivatives and portions of the natural sequence) is a matter which can be determined by routine experimentation.

RNA sequences which are derivatives of the 5'-untranslated leaders of RNA viruses can be arrived at in a number of ways once the sequence of the natural leader is known. For example, an RNA sequence can be derived from the natural leader sequence or an effective portion thereof by simple substitution of one base by another (for instance the transversion of a C to a G). Base substitutions can be made one or more times in the natural leader sequence including the case where a particular substitution, in this instance a C to G transversion, is made on a systematic basis throughout the natural leader sequence. Such a systematic substitution may, in appropriate cases, produce an RNA sequence which is derived from the natural leader sequence in the sense that it still retains significant sequence features from the natural leader sequence.

Other derivatives are those sequences which embody motifs which are especially characteristic of the natural 5'- untranslated leader sequence. A particular example is a leader synthesised to be of a comparable length to the Ω' sequence of TMV, but composed of a motif which is characteristic of Ω', namely the sequence (CAA) The frequency of occurrence of the cyclic permutations of this ordered set of the three bases in Ω' is nearly three times the frequency that would be expected of random sequences having the same overall composition as Ω'.

The 5'- leader sequences of RNA viruses (or derivatives or portions thereof) can be used as such as translational enhancers according to the invention by ligating them upstream of an appropriate mRNA using for example T₄ RNA ligase. The 5'- leader sequence may be immediately adjacent to the initiation codon of the downstream mRNA, or it may be spaced therefrom by an intervening sequence. However, it is preferred to use the 5'- leader sequence in the form of complementary DNA. The cDNA can be obtained in any manner which is desired. However in view of the relatively short length of the 5'- leader sequences in question it will usually be most convenient to synthesise the cDNA chemically. The cDNA will generally be incorporated into an expression vector. An expression vector is any vector capable of expressing those DNA sequences contained therein which are operably linked to other sequences capable of effecting their expression. An expression vector will usually contain a DNA sequence comprising a promoter, the sequence complementary to the 5'- leader sequence of an RNA virus up to but not including the first initiation codon of the viral RNA (or a derivative or portion thereof) and an open reading frame including an appropriate initiation codon. The open reading frame will generally code for one or more proteins of interest. The expression vector may be introduced directly into the host cell stably or transiently where the DNA sequence is expressed by transcription and translation. In the case of stable expression the vector must be replicable in the host either as an episome or as an integral part of the chromosomal DNA. Alternatively the expression vector can be transcribed in vitro and the RNA introduced directly into the cell for transient expression.

The translational enhancers according to the invention can be used in any situation where it is desired to enhance the translational efficiency of mRNA. However it should be noted that the effect of the enhancers is most marked in the case of an mRNA which is otherwise poorly translated. Thus the addition of either a cap or an enhancer according to the invention or both will stimulate translation. In the case of an mRNA already bearing a cap the effect of the enhancer according to the invention, is still present.

The translational enhancers according to the invention are effective in many different types of expression system, for example plant cells, bacteria and animal cells although a particular enhancer may be more suitable for one system rather than another. For example during testing in vitro the omega sequence of TMV appears to show a greater degree of enhancement in the wheat germ system and E. coli than in the rabbit reticulocyte lysate system although enhancement of translation is observed in all three systems. The matching of particular enhancers to particular expression systems will generally be a matter well within the expertise of the man of ordinary skill.

In general the translational enhancers according to the invention can be used with advantage in any case where the value of an organism or cell line to commerce, including agriculture, is determined by the level of expression of one of its natural genes or of an artifically introduced gene.

(i) In the manufacture of commercially useful proteins in genetically engineered organisms or cell lines.

(ii) In fermentation where the rate of production of a product of intermediary metabolism catalysed by enzymes can be enhanced by increasing the rate of synthesis of a particular enzyme.

(iii) In viral cross-protection where the effects of a viral infection can be reduced by prior exposure to an attenuated form of the virus and it is advantageous to increase the level of protection by increasing the level of production of the component which provides the protection. For example plants can be protected against infection by TMV by exposing them to a genetically engineered strain of Agrobacterium which contains a cDNA copy of that portion of the TMV RNA which codes for the viral coat protein. Bacterial cDNA inserted in the plant genome continually expresses the coat protein which protects the plant against infection by the virus itself.

(iv) In the genetic engineering of plants where a property such as herbicide resistance is installed by artificial modification of the intermediary metabolism of the plant.

(v) In the expression of a partial clone of a gene, for example where it is desired to raise antibodies to the product of expression of the partial clone.

The invention is illustrated further by the following experimental work.

EXPERIMENTAL

mRNA

Chimaeric mRNA according to an embodiment of the invention was produced in the following manner, as illustrated in FIG. 1.

Using the sequence of TMV RNA vulgare strain (Goelet et al., Proc. Natl. Acad. Sci. USA 79 5818 (1982)), an 81 base oligonucleotide was synthesised containing the DNA sequence complementary to the 67 base sequence upstream of the AUG initiation codon of the 126 kDa open reading frame, but not including the 5' cap (m⁷ Gppp and adjacent G residue). This sequence is called omega prime. A HindIII site was incorporated at the 5' end and a Sal I site at the 3' end of this oligonucleotide. A second oligonucleotide of 20 bases, including 13 bases at the 3' end of the omega prime sequence and a Sal I site, was synthesized as a primer for the synthesis of the second strand. After annealing, second strand synthesis was completed using the Klenow fragment of DNA polymerase I.

The resulting double-stranded 81 bp fragment was digested with HindIII/Sal I to create a 77 bp fragment with single-stranded cohesive ends and inserted into the corresponding sites of pUC19. The fact that the synthesis had resulted in the correct orientation of the omega prime sequence of TMV was verified by sequence analysis using the dideoxy chain termination method (Sanger et al., Proc. Natl. Acad. Sci. USA 74, 5463 (1977)). The fragment containing omega prime of TMV was then introduced into the HindIII/Sal. I sites of plasmid pJII1 to result in pJII101. pJII1is derived by inserting the TMV origin of assembly sequence as a BamHI fragment into the corresponding site of pSP64 (Melton, Nuc. Acids Res. 12 7035 (1984)) and modifying the SmaI site to a Bgl II site (Sleat et al. Virology 155, 299 (1986)).

A 779 bp Sal I ended fragment from pCM1 (Close and Rodriguez, Gene 20, 305 (1982)) containing the chloramphenicol acetyltransferase gene (CAT) of Tn9 was introduced into the Sal I site of pJII1 and pJII101, resulting in pJII2 and pJII102, respectively. A 1.2 kb Bgl II/Sal I fragment from pKNR-H (Gallie et al., J. Bacteriol. 161, 1034 (1985)) containing the neomycin phosphotransferase II gene (NPTII) of Tn5 was made blunt ended with Klenow fragment and introduced into the Acc I site of pJII1 and pJII101 to result in pJII3 and pJII103, respectively. Correct orientation of both the CAT and NPTII genes was verified by restriction fragment analysis.

After linearisation of pJII2 and pJII102 with BglII and pJII3 and pJII103 with EcoRI, RNA transcripts of each were made using SP6 RNA polymerase. Capped versions of each were also made using GpppG and SP6 polymerase reaction conditions which ensure comparable yields of capped transcripts. Equivalent yields of RNA from the capped and uncapped reactions and for the constructs with or without the omega sequence were verified by denaturing formaldehyde-agarose gels.

Equivalent amounts of the various CAT containing transcripts and the NPTII containing transcripts were added to the rabbit reticulocyte lysate (MDL), wheat germ S-30 (WG) and E. coli S-30 in vitro translation systems under standard reaction conditions. The resulting polypeptide products were labelled with ³⁵ S-methionine.

FIGS. 2 and 3 show the SDS-PAGE analysis of the translation products from MDL (FIG. 2, tracks 1 to 11), WG (FIG. 2, tracks 12 to 22) and E. coli (FIG. 3). In FIG. 2 template additions were: tracks 1 and 12, H₂ O (endogenous translation); tracks 2 and 13, BglII-linearized pJII102 DNA; tracks 3 and 14, CAT RNA; tracks 4 and 15, omega prime-CAT RNA; tracks 5 and 16, 5'-capped CAT RNA; tracks 6 and 17, 5'-capped-omega prime-CAT RNA; tracks 7 and 18, EcoRI-linearized pJII103 DNA; tracks 8 and 19, NPTII RNA: tracks 9 and 20, omega prime-NPTII RNA; tracks 10 and 21, 5'-capped NPTII RNA; tracks 11 and 22, 5'-capped-omega prime-NPTII RNA. ¹⁴ C-labelled marker proteins (Amersham International, plc) were loaded on track M. Their respective sizes in kilodaltons (kDa) are shown on the left. The dried gel was autoradiographed for 3 days at room temperature. In FIG. 3 template additions were: track 1, H₂ O (endogenous translation); track 2, BglII-linearized pJII102 DNA; track 3, CAT RNA; track 4, omega prime-CAT RNA; track 5, 5'-capped CAT RNA; track 6, 5'-capped-omega prime-CAT RNA; track 7, ECoRI-linearized pJII103 DNA; track 8, NPTII RNA: track 9, omega prime-NPTII RNA; track 10, 5'-capped NPTII RNA; track 11, 5'-capped-omega prime-NPTII RNA. ¹⁴ C-labelled marker proteins were loaded on track M and their sizes (in kDa) are shown on the left. The dried gel was autoradiographed for 3 days at room temperature. It should be noted that although the E. coli S-30 system is a transcriptionally active system, the native E. coli RNA polymerase does not recognise the SP6 promoter.

The following Table 1 shows the stimulating effect of the omega prime sequence. Zones from gels shown in FIGS. 2 and 3 were excised and the radioactivity in each band of product of translation was measured and the data normalised in each case to that from RNA not containing the omega prime sequence. Expressing the data in this way eliminates any stimulating effect of the cap alone and shows only the stimulating effect of the omega prime sequence.

                  TABLE 1                                                          ______________________________________                                         mRNA              WG       MDL     E. coli                                     ______________________________________                                         CAT               1        1       1                                           omega prime-CAT   10       1.2     11                                          GpppG-CAT         1        1       1                                           GpppG-omega prime-CAT                                                                            3.8      2.2     2.7                                         NPTII             1        1       1                                           omega prime-NPTII 71       17.8    74                                          GpppG-NPTII       1        1       1                                           GpppG-omega prime-NPTII                                                                          9.5      1.9     9.6                                         ______________________________________                                    

DISCUSSION

The presence of the omega prime region of TMV stimulates the expression of transcripts containing either the CAT or NPT II sequences in either the capped or uncapped state. Stimulation was most marked with the translationally inefficient NPTII transcripts. Thus, the Tn5 fragment containing the NPTII gene contains an additional AUG immediately upstream from the AUG start codon of the NPTII gene. In addition, the "Kozak rules" (see the papers by Kozak referred to above) would suggest that the region about the NPTII start codon constitutes a poor signal for translational initiation. As seen in WG and MDL (FIG. 2, lanes 8 and 19), the NPTII transcript without either a cap or omega prime functions poorly. The addition of a cap or omega prime improves translation although the presence of omega prime has a greater stimulatory effect than the presence of a cap alone. E. coli translational machinery, although not cap-dependent, is greatly stimulated by the presence of the omega prime sequence (FIG. 3, lanes 4, 6, 9 and 11). In E. coli, addition of a cap to the omega prime sequence on CAT or NPTII mRNA did little to enhance its translation whereas addition of omega prime to capped CAT or NPTII mRNA resulted in greater enhancement.

CAT gene mRNA, even without a cap or the omega prime sequence, functions more efficiently in the eukaryotic systems. The first AUG from the 5' terminus is the start codon for CAT and the sequence in and about the start codon conforms well to the "Kozak rules". This is most evident in the MDL system where a much lower mRNA concentration was required to prevent exhaustion of the available ³⁵ S-methionine pool. Not surprisingly then, the presence of the omega prime sequence in CAT mRNA was less stimulatory. MDL is thought less cap-dependent than WG, however the presence of a cap on the NPTII transcript had a slight stimulatory effect (FIG. 2, lanes 8 and 10), whereas its presence on the CAT transcript had little effect (FIG. 2, lanes 3 and 5). CAT mRNA alone was much less active in the WG system. As a result, the presence of either a cap or the omega prime sequence or both was highly stimulatory (FIG. 2, lanes 14 to 17). In WG, addition of a cap to the omega prime sequence on NPTII mRNA (FIG. 2, lanes 20 and 22) did little to enhance its translation whereas addition of omega prime to capped or uncapped NPTII mRNA (FIG. 2, lanes 19 and 21) resulted in a greater enhancement.

Therefore the presence of the omega prime sequence enhances translation in both eukaryotic (MDL and WG) and prokaryotic systems. This is consistent with the ability of TMV particles to be uncoated and the RNA translated by eukaryotic or prokaryotic translational machinery. The omega prime sequence often has a greater stimulatory effect on the expression of transcripts than does the presence of a cap alone. In addition, the omega prime sequence enhances translation under conditions where capping does not, e.g. in the prokaryotic and, to some extent, the MDL systems.

FURTHER EXPERIMENTAL WORK

Chimaeric RNA constructs including CAT mRNA and derivatives or portions of the omega prime leader sequence were made in an analogous manner to that described above, the initial synthetic DNA sequence being chosen to produce the required RNA. As shown in FIG. 4, there were five deletions (referred to as Δ1 to Δ5), one A→C transversion and one [C,A]_(n) →[U]_(n) substitution.

Further chimaeric RNA constructs containing CAT mRNA were made in an analogous manner with 5'-leader sequences of viruses other than TMV (U1, vulgare, or common strain), namely the tomato strain of TMV (SPS isolate), Turnip Yellow Mosaic Virus (TYMV), Brome Mosaic Virus RNA 3 (BMV 3), Rous Sarcoma Virus partial leader (RSV) and Alfalfa Mosaic Virus RNA 4 (A1MV4). The initial DNA sequences synthesised and restriction site diagrams are shown respectively in FIGS. 5 to 9.

A further set of RNA constructs was prepared in an analogous manner, but with the E. coli reporter gene β-glucuronidase (GUS) in place of CAT. The leader sequences used were either polylinker (for comparison purposes) or U1 omega (i.e. omega prime from TMV) or the derivatives and portions of omega prime and other viral leaders as described above. The GUS gene was used either in an artificial "good" Kozak context (see p. 33. re. Table 9) or in the native "bad" Kozak context as a SalI fragment from pRAJ235/240 (Jefferson et al. Proc. Natl. Acad. Sci. 83 8447 (1986)). The leader sequences and restriction site diagrams are shown in FIGS. 10 to 13. The expression of the reporter gene was determined fluorimetrically as described by Jefferson et al. Proc. Natl. Acad. Sci. 83 8447 (1986).

In addition to the three in vitro translation systems described above, two in vivo systems were used: microinjection of Xenopus laevis oocytes and electroporation of tobacco (Nicotiana tabacum cv Xanthi) mesophyll protoplasts. The respective techniques were as follows:

Two ng of each synthetic SP6 mRNA (or their corresponding DNA templates) were microinjected into the cytoplasm of stage 6 X. laevis oocytes in batches of 30 using standard procedures (Colman A. In Hames, B. D. and Higgins, S. J. (eds.), Transcription and Translation: A Practical Approach, IRL Press, Oxford. 271 (1984)). Oocytes were incubated for 18 hours in Modified Barths' Saline (MBS), then washed briefly in distilled water. Extracts from Xenopus oocytes were prepared by resuspending each sample in 0.25M Tris-HCl, pH 7.4, containing 10 mM DTT (17 μl/oocyte), followed by sonication for 15 sec. Insoluble material was removed by microcentrifugation for 15 min. and fractions of the resultant extract representing equivalent numbers of oocytes were assayed for CAT or GUS activity by published procedures (CAT: Gallie et al. Nuc. Acids. Res. 15 3257 (1987); GUS: Jefferson et al. Proc. Natl. Acad. Sci. 83 8447 (1986)).

Mesophyll protoplasts were isolated from leaves of N. tabacum (cv. Xanthi) and stored in 0.7M mannitol. The protoplasts (10⁶) were centrifuged at 60 xg for 3 min, resuspended in 1 ml ice-cold 0.7M. mannitol containing the RNA (8 μg), transferred to the electroporation cell and given a single voltage pulse (2.5 kV/cm) by discharging a 50 nF capacitor through the cell. The pulse had a rapid rise-time (less than 1 μs) and an exponential decay with a half-life of about 5 μs. Electroporated protoplasts were stored at 0°-4° C. for 10 min. Ten mls of 0.7M mannitol were added and the protoplasts recovered by centrifugation at 60 xg for 3 min. The protoplasts were then resuspended in culture medium and incubated at 25° C. for 21 hours in 9 cm-diameter Petri dishes.

Each sample of electroporated protoplasts was sedimented, resuspended and sonicated in 150 μl of 0.25M Tris-HCl, pH 7.4 containing 2 mM leupeptin and 10 mM dithiothreitol (DTT) for CAT assays or 50 mM sodium phosphate pH 7.0, 10 mM 2-mercapto ethanol for GUS assays. Extracts were microcentrifuged at 10,000 xg for 10 min at room temperature or 4° C.

FIG. 14 shows the effect of omega prime on the expression of capped or uncapped CAT RNAs in electroporated tobacco protoplasts. Each origin spot received extract equivalent to 5'10⁴ viable protoplasts. For each sample, the conversion (%) of substrate (¹⁴ C-chloramphenicol) into its mono-acetylated forms (indicated on the left) is shown at the top of the track. Electroporated RNAs or standards were: track 1, no RNA (mock); track 2, CAT RNA; track 3, omega prime-CAT RNA; track 4, 5'-capped CAT RNA; track 5, 5'-capped-omega prime-CAT RNA; track 6, 0.1 unit purified CAT enzyme added to an equivalent volume of extract as for track 1; track 7, 0.1 unit purified CAT enzyme alone. The dried tlc plate was autoradiographed for 2 days before removing and determining the radioactivity in the relevant ¹⁴ C-labelled spots of mono-acetylated products (3-Ac/1-Ac), % conversion data reflects the relevant CAT activity in the extract. Omega prime (+/-a 5'-cap) was the most stimulatory of CAT RNA expression.

FIG. 15 shows the effect of omega prime on the expression of capped or uncapped CAT RNAs microinjected into X. laevis oocytes. An equal amount of each CAT RNA-construct or linearized DNA template (2 ng) was injected per oocyte, throughout. Equal oocyte extract volume (equivalent to 0.3 x cell) were assayed in each case, unless stated otherwise. Conversion (%) of ¹⁴ C-chloramphenicol into its mono-acetylated forms, is shown at the top of each track. In tracks 5 and 6, conversion (%) into the di-acetylated form is indicated by *. Microinjected RNAs or standards were: track 1, no RNA (mock); track 2, BglII-linearized pJII102; track 3, CAT RNA; track 4, omega prime-CAT RNA; track 5, 5'-capped CAT RNA; track 6, 5'-capped-omega prime-CAT RNA; track 7, as for track 5 but 20-fold diluted extract; track 8, as for track 6 but 20-fold diluted; track 9, 0.1 unit purified CAT enzyme added to an equivalent volume of extract as in track 1; track 10, 0.1 unit purified CAT enzyme alone. The dried tlc plate was autoradiographed at room temperature for 18 hours, before excising and counting the relevant ¹⁴ C-labelled spots.

The following Tables show the further results obtained.

                  TABLE 2                                                          ______________________________________                                         Effect of deletions and other sequence alterations on                          translational enhancement by omega prime. Chloramphenicol                      acetyltransferase expression in RNA-microinjected Xenopus                      oocytes.                                                                                          % conversion of .sup.14 C-                                                     chloroamphenicol to the 1-                                  RNA injected       and 3- monoacetylated forms                                 ______________________________________                                         Nil (mock)         0.1                                                         CAT                4.5                                                         Omega prime CAT    34.2                                                        Δ1 - CAT     17.2                                                        Δ2 - CAT     9.7                                                         Δ3 - CAT     17.6                                                        Δ4 - CAT     14.6                                                        Δ5 - CAT     17.9                                                        A -> C transversion - CAT                                                                         15.7                                                        [C,A].sub.n -> [U].sub.n substitution - CAT                                                       34.3                                                        0.1 units CAT (pure enzyme)                                                                       1.2                                                         ______________________________________                                          All RNA's were uncapped                                                  

The above results show that omega prime enhances translation of CAT. Deletions 1 to 5 reduce this effect without destroying it entirely. The same is true of the A→C transversion. The [U]_(n) substitution has little effect, if any.

                  TABLE 3                                                          ______________________________________                                         Effect of untranslated 5'-leader sequences from                                different viruses on the level of expression of                                chloramphenicol acetyltransferase after microinjection of                      mRNA into Xenopus oocytes.                                                                       % conversion of .sup.14 C-                                                     chloroamphenicol to the 1-                                   RNA injected      and 3- monoacetylated forms                                  ______________________________________                                         Nil (mock)        0.1                                                          CAT               6.3                                                          Omega U1 CAT      46.5                                                         Omega (SPS) CAT   35.8                                                         TYMV CAT          1.3                                                          BMV3 CAT          5.0                                                          RSV CAT           5.4                                                          A1MV4             6.7                                                          0.1 Units CAT (pure enzyme)                                                                      1.3                                                          ______________________________________                                          All RNAs were uncapped                                                         Omega U1 derives from the standard strain of TMV (Vulgare).                    Omega SPS is from the tomato strain of TMV.                                    TYMV: turnip yellow mosaic virus leader.                                       BMV3: brome mosaic virus RNA3.                                                 RSV: a partial leader sequence from Rous Sarcoma virus.                        A1MV4: alfalfa mosaic virus RNA4.                                        

The above results show that omega prime from TMV Ul enhances translation of CAT RNA in this system. TMV tomato strain omega prime also does so, but a little less well. Other viral leaders have little or no effect, TYMV leader may be inhibitory.

                  TABLE 4                                                          ______________________________________                                         Effects of different 5'- non-translated leader sequences                       with or without a 5'-cap on in vitro translation of                            chloramphenicol acetyltransferase mRNA in a wheat germ cell-                   free system. (The results as shown below have been                             normalized to the unmodified CAT mRNA leader sequence which                    was taken as 1.0 to show fold-stimulation over CAT mRNA                        alone).                                                                        RNA added            Uncapped  capped                                          ______________________________________                                         CAT                  1.0       1.0                                             Omega prime CAT      2.8       3.7                                             Δ1-CAT         1.2       0.6                                             Δ2-CAT         2.0       1.8                                             Δ3-CAT         1.5       1.3                                             Δ4-CAT         3.0       1.5                                             Δ5-CAT         1.5       0.7                                             A -> C transversion-CAT                                                                             3.3       2.9                                             [C,A].sub.n -> [U].sub.n substitution-CAT                                                           2.1       3.4                                             TMV Tomato strain Omega                                                                             1.7       3.0                                             prime-CAT                                                                      TYMV-CAT             0.3       3.0                                             BMV3-CAT             1.4       2.8                                             RSV-CAT              0.6       2.5                                             A1MV4-CAT            2.1       2.3                                             ______________________________________                                          Note:                                                                          these figures represent comparisons of proportional stimulations of            translation produced by the inclusion of different sequences at the 5' en      of the CAT message. Translations were done with capped or with uncapped        messages. Each column represents the yield of radioactive CAT protein          recovered by excision from an independent gel (as in FIGS. 2 and 3). The       bands were cut out, counted, and have been normalised to a band for which      the mRNA was unmodified CAT message.                                     

The above results show that omega prime enhances in this system, with or without 5'-capping. The other viral leader sequences and derivatives or portions of omega prime enhance translation to varying extents, but rarely significantly more than omega prime itself.

                  TABLE 5                                                          ______________________________________                                         Effects of different non-translated leader sequence,                           all without a 5'-cap structure, on in vitro translation of                     chloramphenicol acetyltransferase mRNA in an E. coli cell-                     free system.                                                                   RNA added             Uncapped                                                 ______________________________________                                         CAT                   1.0                                                      Omega prime CAT       9.0                                                      Δ1 - CAT        3.3                                                      Δ2 - CAT        2.5                                                      Δ3 - CAT        6.3                                                      Δ4 - CAT        4.2                                                      Δ5 - CAT        3.8                                                      A -> C transversion CAT                                                                              4.1                                                      [C,A].sub.n -> [U].sub.n substitution CAT                                                            1.9                                                      TMV Tomato strain Omega CAT                                                                          6.6                                                      TYMV CAT              0.2                                                      BMV3 CAT              3.7                                                      RSV CAT               1.5                                                      A1MV4 CAT             1.4                                                      ______________________________________                                    

The above results show that omega prime, some of its derivatives, and tomato strain omega prime enhance translation to the greatest extent. Most of the other sequences also enhance, but to varying extents. E. coli does not require a 5' cap for translation.

                  TABLE 6                                                          ______________________________________                                         Effect of different leaders on the translation of β-                      glucuronidase mRNA (from .sub.p RAJ235/240) electroporated into                tobacco protoplasts.                                                                           nmoles 4-methyl                                                GUS mRNA        umbelliferyl glucuronide                                                                        Fold                                          construct       cleaved min.sup.-1 /μg pro-                                                                  Stimu-                                        used *          tein in protoplast extract                                                                      lation                                        ______________________________________                                         [Blank](no mRNA)                                                                               --               --                                            Uncapped Leader                                                                235 [SP6 transcript,                                                                           <0.01             1                                            polylinker leader.]                                                            Omega prime (U1)-235                                                                           0.25             >25                                           Omega prime (SPS)-235                                                                          0.15             >15                                           TYMV -235       <0.01            --                                            A1MV4 -235      <0.01            --                                            BMV3 -235       <0.01            --                                            RSV -235        <0.01            --                                            Capped Leader                                                                  capped 235 [SP6 0.03              1                                            transcript polylinker                                                          leader]                                                                        capped Omega prime -235                                                                        0.54             18                                            capped Omega prime                                                                             0.43             14                                            (SPS) -235                                                                     capped TYMV -235                                                                               <0.01            --                                            capped A1MV4 -235                                                                              0.23              8                                            capped BMV3 -235                                                                               0.23              8                                            capped RSV -235 0.21              7                                            capped Δ1 -235                                                                           0.55             18                                            capped Δ2 -235                                                                           0.50             17                                            capped Δ3 -235                                                                           0.32             11                                            capped Δ4 -235                                                                           0.54             18                                            capped Δ5 -235                                                                           0.54             18                                            capped A -> C   0.69             23                                            transversion -235                                                              capped [C,A].sub.n -> [U].sub.n                                                                <0.01            --                                            substitution -235                                                                            (blank value subtracted from all                                               others)                                                          ______________________________________                                          (* Constructs were "bad" GUS ex pRAJ 235/240, with or without a 5' cap)  

All mRNA constructs were electroporated at 8 μg/ml and washed protoplasts incubated for 20 hours as described (Gallie et al Nuc. Acids. Res. 15, 3257 (1987)).

The sequence of the "Bad" Kozak construct (235) leader is as follows: ##STR1## The insertion of Omega prime into the polylinker is as below: ##STR2## and the deleted omega prime derivatives, etc. follow accordingly.

Messenger RNAs encoding calf preprochymosin or chicken prelysozyme were synthesized in vitro, with or without the TMV 5'-terminal sequence (omega prime). The mRNAs were translated in vitro in the systems derived from rabbit reticulocytes (MDL), wheat-germ (WG) or E. coli (EC).

The plasmids pJII6 and pJII7 (FIG. 16) were constructed using components of plasmids pSP64CT and pSP64LT (Sleat et al. Virology, 155 299 (1986)) and pJIII01. Both pJII6 and pJII7 are derivatives of the transcription plasmid pSP64 (Melton, 1984), which contains a promoter for the bacteriophage SP6 RNA polymerase adjacent to the polylinker sequence derived from M13 mp11. The preprochymosin and prelysozyme coding regions of pSP64CT and pSP64LT respectively were excised as HindIII fragments, blunt-ended with the Klenow fragment of DNA polymerase 1, and cloned into the HincII site of pJIII01 to generate pJII6 and pJII7 respectively (FIG. 16). These constructs were used to direct the synthesis of messenger-sense preprochymosin and prelysozyme transcripts bearing a 5'-terminal omega prime sequence. Parental plasmids pSP64CT and pSP64LT were templates for synthesis of mRNAs lacking omega prime.

The results are shown in Table 7 below.

                  TABLE 7                                                          ______________________________________                                         Effect of omega prime on expression of two eukaryotic                          mRNAs in 80S- and 70S-ribosome-based translation systems.                      Incorporation of L-[.sup.35 S]-methionine (cpm) into gel-                      purified polypeptides.                                                                        CPM (Rel. Stimulation)                                          Cell-free system MDL      WG        E. coli                                    ______________________________________                                         SP6 transcript                                                                 Prelysozyme      5040 (1.0)                                                                               3730 (1.0)                                                                               0 (1.0)                                   omega prime prelysozyme                                                                         5290 (1.0)                                                                              10240 (2.8)                                                                              1030 ( )                                   Preprochymosin   2470 (1.0)                                                                               1110 (1.0)                                                                               0 (1.0)                                   omega prime preprochymosin                                                                      7280 (3.0)                                                                              10010 (9.1)                                                                              2180 ( )                                   ______________________________________                                    

In an analogous manner to the methods described above, various chimaeric RNA constructs were prepared with the NPT II coding region, and either omega prime, two tandem omega prime regions, a long (77 base) "junk" leader (polylinker) sequence or "junk" together with a central omega prime. The constructions are shown in FIG. 17 and the results in Table 8below:

                                      TABLE 8                                      __________________________________________________________________________     EFFECT OF VARIOUS 5'-LEADER CONSTRUCTIONS ON IN VITRO                          TRANSLATION OF SP6 TRANSCRIPTS OF                                              NEOMYCIN PHOSPHOTRANSFERASE                                                                   Cell-free Translation System                                                            Rabbit reticul-                                                       Wheat Germ                                                                              ocyte lysate                                                                            E.coli                                                       (-cap)                                                                              (+cap)                                                                             (-cap)                                                                             (+cap)                                                                              (-cap)                                                                             (+cap)                                    __________________________________________________________________________     NPTII          1*   1   1   1    1   1                                         omega prime NPTII                                                                             8.1+ 5.9 1.3 0.8  15.2                                                                               4.8                                       [omega-omega]prime NPTII                                                                      8.2  6.3 1.1 0.7  44.9                                                                               15.0                                      "JUNK"-NPTII   0.9  0.4 0.1 0.1  3.5 2.3                                       "JUNK"-omega prime NPTII                                                                      1.8  1.1 0.3 0.3  9.8 8.2                                       __________________________________________________________________________      *Background (H.sub.2 O or linearized DNA) corrected / All values =             Foldstimulation over NPTII RNA alone.                                    

This shows that a random polylinker sequence will not substitute for omega prime in 80S ribosome systems and only poorly in a prokaryotic (70S) system. Also two tandem omega primes only benefits 70S expression (c.f. single copy of omega prime).

To extend the data presented in Table 6 (above) on the effect of omega prime on expression of the β-glucuronidase mRNA (GUS mRNA), a new series of constructs utilized the GUS gene with a modified-sequence context around the initiation codon (AUG). Plasmid pRAJ275, containing the "good context" leader ##STR3## was the source for this version of GUS. Omega prime and various other leader constructs were inserted at the Hind III/Sal I sites as before. Details of the pRAJ275 construct are as follows:

pRAJ275 is a derivative of pRAJ255 (Jefferson. et al. Proc. Natl. Acad. Sci. 83, 8447 (1986)) in which the 5' sequences of GUS were removed by progressive BAL31 deletion and replaced with a synthetic oligonucleotide that has constructed a "consensus" translational initiator as defined by Kozak (Microbiol. Reviews, 47, 1 (1983)). This plasmid has a unique Nco I site (CCATGG) positioned at the initiator ATG codon, with the surrounding context GTCGACCATGGTC. It has been shown both in vitro and in vivo that the context around the initiator can have a profound effect on the levels of translated product from a given amount of mRNA. This construction has been adjusted to maximise this effect. It was provided as a Sal I-Eco RI fragment in pUC19.

5'-capped or uncapped transcripts were electroporated into tobacco protoplasts as before and the level of GUS expression measured in extracts.

Uncapped RNAs were also added to MDL or WG cell-free translation systems and the level of GUS protein synthesized measured by excising and counting radioactive gel bands as for FIGS. 2, 3 etc. (Table 1). The results are shown in Table 9.

                                      TABLE 9                                      __________________________________________________________________________     Translational Enhancement by omega prime on GUS mRNA                           In vivo translation                                                            Electroporated tobacco protoplasts                                                       Gus activity                                                                   nmoles 4-methyl                                                                umbelliferyl In vitro translation                                              glucuronide  MDL        WG                                                     cleaved min.sup.-1 /μg                                                                   Cpm .sup.35 S                                                                             Cpm .sup.35 S                                          protein in                                                                              Fold                                                                               methionine-                                                                           Fold                                                                               methionine-                                                                           Fold                                  Kozak     protoplast                                                                              stim-                                                                              labelled GUS                                                                          stimu-                                                                             labelled GUS                                                                          stimu-                                context   extract  ulation                                                                            in gel bands                                                                          lation                                                                             in gel bands                                                                          lation                                __________________________________________________________________________     Uncapped                                                                       RNAs                                                                           GUS 235                                                                              bad <0.01    (1) 7180   (1.0)                                                                              2400   (1.0)                                 (40b poly-                                                                     linker                                                                         leader)                                                                        Omega bad 0.18     (>18)                                                                              19700  (2.7)                                                                              9720   (4.1)                                 prime-235                                                                      GUS 275                                                                              good                                                                               <0.01    (1) 12920  (1.0)                                                                              3010   (1.0)                                 (28b poly-                                                                     linker                                                                         leader)                                                                        Omega good                                                                               0.35     (>35)                                                                              29280  (2.3)                                                                              19720  (6.6)                                 prime-275                                                                      5'capped                                                                       leader                                                                         GUS 235                                                                              bad 0.03     (1)                                                         (40b poly-                                                                     linker                                                                         leader)                                                                        Omega bad 0.61     (20)                                                        prime-235                                                                      GUS 275                                                                              good                                                                               0.04     (1)                                                         (28b poly-                                                                     linker                                                                         leader)                                                                        Omega good                                                                               3.2      (80)                                                        prime-275                                                                      __________________________________________________________________________      Again in all cases, both in vitro (MDL,WG) and in vivo (protoplasts) with      5'capped or uncapped GUS mRNAs, with good or bad Kozak context around the      initiation codon, omega prime enhances mRNA expression significantly.    

A further set of RNA constructs was prepared, in an analogous manner to the method described above, with the E. coli reporter gene β-glucuronidase (GUS) and additionally the inducible trytophan (Trp) promoter. The Trp promoter was purchased from Pharmacia Ltd., and modified by conventional techniques of restriction endonuclease digestion, filling-in, sub-cloning etc, to create a 3'-Hind III site (FIG. 18) and a blunt 5'-end. This dsDNA fragment was then inserted into the various leader-reporter (GUS) gene plasmids between the SP6 promoter (silent in E. coli in vivo) and the 5'-leader construct of interest.

The resulting plasmid DNA was used to transform competent E. coli cells by standard techniques and the Trp promoter induced in situ to produce chimaeric leader-reporter mRNAs. Transcription start point corresponds to the G in the Hind III site (AAGCTT).

Expression of the transcripts in transformed E. coli cells was determined spectrophotometrically (Jefferson et al. Proc. Natl. Acad. Sci. 83 8447 (1986)). Results (Table 10) indicate that expression of GUS was enhanced in vivo from either the "good" or "bad" Kozak context construct by the presence of omega prime and to some extent by the other leader sequences or derivatives of omega prime.

                  TABLE 10                                                         ______________________________________                                         Expression controlled by the Trp promoter in stably                            transformed E. coli cells                                                                         Approximate relative                                                           spectrophotometric                                                             rates of conversion of                                                         p-nitrophenyl                                                                  glucuronide                                                 ______________________________________                                         235-GUS ("Bad" context)                                                                             1                                                         omega prime 235 GUS  7x                                                        275-GUS ("Good" context)                                                                            1                                                         omega prime-275-GUS ("Good"                                                                         8x                                                        context)                                                                       Δ1-275-GUS     5-8x                                                      Δ2 275 GUS     (75x: To be confirmed)                                    Δ3 275 GUS     (75x: To be confirmed)                                    Δ4 275 GUS     5-8x                                                      Δ5 275 GUS     5-8x                                                      A -> C 275 GUS       (75x: To be confirmed)                                    [C,A].sub.n -> [U].sub.n 275 GUS                                                                    5x                                                        TYMV 275 GUS         3-4x                                                      A1MV4 275 GUS        3-4x                                                      BMV3 275 GUS         1-2x                                                      RSV 275 GUS          1-2x                                                      ______________________________________                                          (The results above were normalized to the respective GUS construct with        the polylinker leader alone.                                             

Conclusions:

Omega prime enhances the expression of GUS RNA in vivo in E. coli transformed with plasmid DNAs, using Trp promoter in situ.

Further experiments were performed using supercoiled double stranded pUC19 DNA which is modified to contain the cauliflower mosaic virus (CaMV) 35S promoter, the nopaline synthase terminator (from Agrobacterium tumefaciens) and in between them either CAT DNA (, ▴) or omega prime CAT DNA (∘, Δ).

The constructs (FIG. 20) were prepared from a binary vector system (Bevan, Nuc Acids Res. 12 8711 (1984)) wherein the CaMV 35S promoter and the Nos terminator were excised using Hind III and Eco RI sites and inserted into plasmid pUC19 thus giving pUC35S Nos.

The constructs pUC35SCATNos and pUC35S omega prime CATNos were made by inserting either CAT or omega prime CAT respectively into the plasmids using the Bam HI. site.

Various levels of either plasmid DNA (in μg/ml) were electroporated into protoplasts as before, and the cells incubated at 25° C. for 6 hours (∘,) or 26 hours (Δ,▴) and lysed. CAT assays (as before) were carried out.

The results as seen in FIG. 20 indicate that cells which received 100 μg-150 μg of supercoiled plasmid DNA showed enhanced CAT expression (2 fold) when omega prime is present in the construct (Δ,∘).

These results show that gene expression in stable transgenic plants will also likely be enhanced by the inclusion of the omega prime sequence between the promoter (for transcription in vivo) and the translational open reading frame.

Derivatives of the Ω' sequence of TMV

The approach described above of cloning conveniently tailed oligonucleotides into polylinker sites of SP6 or trp promoter plasmids creates transcripts with a variety of extra nucleotides at the 5'-end. To approximate more closely to the true relationship between the leader and the coding sequence to be expressed, long synthetic oligonucleotides containing the bacteriophage T7 promoter immediately adjacent to the leader were cloned, via suitable terminal restriction sites, in pUC19. In addition, each untranslated leader sequence terminated with CC to create a NcoI site so that no extra nucleotides were present between the leader and the reporter gene sequence. This approach was applied to three short repetitive sequences found in the TMV Ω' sequence at frequencies greater than the predicted frequency based on random expectation and amplified as cDNA to 65 residues. These derivatives of the TMV Ω' sequence were placed between a T7-promoter-G⁺¹ and NcoI site, cloned and tested for their effect on mRNA expression.

Single-stranded oligo-deoxyribonucleotides corresponding to the RNA polymerase template strand for each leader were synthesised. In addition to the leader sequence, each synthetic oligonucleotide contained a 5'-BamH1 site, a 5'-proximal NcoI site (overlapping through one C-residue), a 3'-proximal T7 promoter and-a 3'-EcoRI site. Thus the extensive common sequence at the 3'-end of all constructs required only the second strand primer [5'-GGAATTCTTAATACGACTCAC-3'] to initiate synthesis of each complementary strand by the Klenow fragment of E. coli DNA polymerase I. Double stranded DNA was digested with EcoRI and BamHI, then ligated into pUC19 via these restriction sites. The GUS gene was isolated from pJII140 (see above) by digestion with NcoI and SalI, gel purified and ligated into the various pUC19 leader plasmids via these same sites.

Each pUC19-leader-GUS plasmid was purified by CsCl gradient centrifugation from large scale cultures of E. coli JM83 recA⁻ according to standard methods (Sambrook et al, Molecular Cloning: A Laboratory Manual (2nd Edition), Cold Spring Harbor Laboratory (1989)). Purified template DNA was linearised by digestion with PstI and T7 transcription performed at 40° C. for 4 hours in 40 mM Tris-HCl, pH 7.5 containing: 6 mM MgCl₂, 2 mM spermidine, 10 mM dithiothreitol, 500 μM each of ATP, CTP, UTP and GTP, 25 U RNAguard (Pharmacia-LKB Inc.), 100 U T7 RNA polymerase and 2 μg of each template DNA. To produce 5'-capped RNAs, the concentration of GTP was reduced to 250 μM and 750 μM GpppG (Pharmacia-LKB Inc.) was included. Before in vitro translation, RNA yield was estimated by ethidium bromide staining after MOPS-formamide-agarose gel electrophoresis (Sleat et al, Gene, 60, 217-225 (1987)).

Transcripts were translated in the message-dependent rabbit reticulocyte lysate (MDL), wheat germ extract (WG) and E. coli (EC) translation systems which are described above. Incorporation of L-[³⁵ S]-methionine into trichloroacetic acid insoluble polypeptides was estimated (Gallie et al, Nucleic Acids Research, 15, 3257-3273, (1987)). Polypeptides were also separated by electrophoresis in 15% (w/v) polyacrylamide gels (Laemmli, Nature, 227, 680-685 (1970)), fixed, dried, autoradiographed and quantified by scanning the X-ray film with a Joyce-Loebl Chromoscan 3 densitometer operating at 530 nm.

Three repetitive motifs of different lengths found in the TMV Ω' sequence at frequencies significantly higher than the expected frequency based on random expectation were amplified to 65-residues to provide the derivatives of the Ω' sequence. The translational effects of these derivatives of the Ω' sequence were compared with GUS mRNAs transcribed from SP6 promoter plasmid pJII139 (minus Ω') and pJII140 (plus Ω') which are both described above. The precise sequences of the leaders derived from the Ω' sequence of TMV are set out below:

Leader "HR1": G (CAA)₂₁ CACCAUGG

Leader "HR2": G(CA)₃₂ CCCAUGG

Leader "HR3": G (ACAAUUAC)₆ CCAUGG

The sequences derived from the plasmids pJII139 and pJII140 are shown in FIGS. 12 and 13 respectively.

Each T7 leader GUS mRNA, as well as SP6 RNAs from pJII139 and pJII140 (+/-Ω') were quantified by agarose gel electrophoresis and added (5'-capped or uncapped) to the different cell free translation systems under standard conditions at 40 μg T7-transcript/ml. The resulting incorporations of L-[³⁵ S]-methionine into trichloroacetic acid insoluble polypeptide (GUS) are summarised below in Table 11. To simplify data comparisons, values are expressed relative to the cpm incorporated by mRNA from pJII140 (Ω'-GUS mRNA=1.00). The absolute cpm for the latter are also shown for reference.

                  TABLE 11                                                         ______________________________________                                         Stimulation* of GUS mRNA expression in vitro by various                        leaders compared to Ω' (= 1.00)                                          Leader                                                                         (see     MDL         WG          EC                                            above)   -CAP    +CAP    -CAP  +CAP  -CAP  +CAP                                ______________________________________                                         HR1      1.29    2.06    1.61  1.26  2.00  1.70                                HR2      0.50    2.16    0.43  0.74  0.08  0.21                                HR3      0.57    1.79    0.94  0.94  0.18  0.33                                pJII139  0.36    0.62    0.36  0.52  0.07  0.30                                Absolute cpm                                                                            3.7     1.9     1.1   3.9   11.7  4.9                                 (× 10.sup.4) per                                                         μl of incuba-                                                               tion with                                                                      pJII140                                                                        mRNA                                                                           ______________________________________                                          *Expressed as TCAinsoluble cpm incorporated by each T7 LeaderGUS mRNA          relative to cpm with pJII140 (Ω') mRNA                             

The above experiments were repeated in MDL using Ω'-CAT mRNA transcribed from plasmid pJII102 (see above and FIG. 1) and mixed with a particular T7-leader-GUS mRNA. Both mRNAs were present at several sub-saturating concentrations (1.5, 3.9 or 7.5 pmol each/ml incubation). The object was to use the Ω'-CAT mRNA as an internal control to confirm the comparability of each in vitro translation reaction. The results were essentially similar to those shown in Table 11 above, with regard to the relative efficacy of each T7-leader-GUS mRNA construct. The levels of CAT expression (as judged by scanning densitometry of the resulting autoradiographs) remained constant and proportional to the level of input mRNA, confirming that the in vitro translations gave reliable measures of mRNA efficiency.

The conclusion that can be drawn from these experiments is that leader HRl (a derivative of Ω' based on the repeating motif CAA) shows a translation enhancing effect consistently superior to that of Ω' itself in all three cell free translation systems. Leaders HR2 and HR3 (derivatives of Ω' based on the repeating motifs CA and ACAAUUAC respectively) also show a considerable translation enhancing effect in the MDL and WG systems which may be comparable to or, in some cases, even greater than that of Ω'. 

We claim:
 1. A method of producing a polypeptide comprising translation of an RNA which encodes said polypeptide, wherein said RNA comprises two operatively linked subsequences, comprisingi) a first subsequence comprised of the Ω' sequence of tobacco mosaic virus, and ii) a second subsequence downstream of said first subsequence and comprised of a coding sequence heterologous to said virus; wherein said first subsequence is capable of enhancing translation of said second subsequence.
 2. A method of producing a polypeptide wherein said method comprises expressing a DNA which encodes said polypeptide, wherein said DNA comprises two operatively linked subsequences comprising:i) a first subsequence comprised of a sequence complementary to the Ω' sequence of tobacco mosaic virus, and ii) a second subsequence downstream of said first subsequence and comprised of a coding sequence heterologous to said virus; wherein in mRNA produced by transcription of said first and second subsequences, mRNA complementary to said first subsequence is capable of enhancing translation of mRNA complementary to said second subsequence. 